Aluminum (Al) or Al alloy has been widely used in the past as the conductive material in silicon semiconductor integrated circuits (LSI). Copper (Cu) has come to be used as the conductive material in order to increase reliability and reduce wiring resistance in the wiring in conjunction with advances in LSI miniaturization.
Since processing by dry etching is difficult, a damascene method is generally used when the copper wiring is formed. The damascene method is a method whereby copper is filled into a wiring trench and/or a via formed in an insulation film on the semiconductor substrate, and wiring is formed by polishing off and removing the excess copper.
The dimensions of wiring have become smaller in recent years in conjunction with LSI miniaturization, and disconnection defects in copper wiring are a serious problem. Copper migration is a contributing factor to wiring disconnection defects. Copper migration can be broadly classified into two types according to drive power. One type is electromigration, which is caused by current flowing in the wiring, and the other type is stress migration, which is caused by stress in the wiring. The current density flowing in the wiring increases as the wiring becomes smaller, and migration of copper easily occurs due to electric wind. A stress gradient occurs at the minute junction of a via and the wiring, and migration of copper occurs in order to alleviate the stress.
Copper migration easily occurs at copper grain boundaries and the interface of different types of material between the copper and the surrounding film. The reason for this is that the activation energy of copper diffusion at these locations is smaller than that of the copper volume diffusion. In order to prevent diffusion of copper, the stability of the copper grain boundaries must be increased, and the adhesion between the copper and the surrounding film must be enhanced.
A method for adding an additive element to the copper wiring to create a copper alloy has been reported as a method for preventing copper migration. In Patent Document 1, an alloy in which silver or the like is added to copper is used as a copper alloy. Methods for forming a copper alloy film in which these elements are added include a sputtering method that uses a target as an alloy to which the additive is added, a method for forming alloys from platings of tin or chromium with copper, and a method for forming an alloy by a CVD method or the like.
There is also a method whereby a film having a function for enhancing adhesion with copper is provided to a barrier metal part. In Patent Document 2, a technique is disclosed for forming a film composed of a material having good adhesion with copper on a barrier metal that is formed by chemical vapor deposition or ALD, whereby electromigration defects and the like are prevented, and the reliability of the wiring is enhanced.    Patent Document 1: Japanese Laid-open Patent Application No. 9-289214    Patent Document 2: Japanese Laid-open Patent Application No. 2003-332426